This disclosure relates generally to proximity warning systems at work sites, and in particular to an interactive magnetic marker field and proximity warning system. Many methods have been devised to warn people against being struck, pinched, crushed or otherwise harmed by vehicles and mobile equipment. Such vehicles and mobile equipment may be used for above and below ground operations. Examples of the equipment include: road construction equipment such as trucks, road graders, rollers and pavers; surface mining equipment, such as for use with gravel and sand operations, front end loaders, trucks, dozers, conveyors and other items; underground mining equipment such as continuous miners, shuttle cars, conveyors, crushers, load-haul-dump vehicles, man-trips, tractors, and other items. The equipment also includes forklifts, cranes, and trucks used at warehouses and shipping ports.
Many people have been killed in the U.S. by such equipment. Unfortunately, the systems that have been devised to help protect people and property in these industrial operations, such as proximity warning and collision avoidance systems, have usually not been very effective. A new proximity warning system was developed and successfully demonstrated for use on continuous miners, as disclosed in U.S. Pat. No. 7,420,471. An objective of the '471 patent is to help prevent the crushing or pinning of personnel who are remotely controlling a continuous miner, and to help protect other personnel assisting in use of the continuous miners. The '471 patent also envisions to provide protection to personnel from other types of mobile equipment and machines. The system of the '471 patent employs a magnetic marker field and an active architecture that incorporates two-way communication between the worker and the machine the worker is near. Warnings are given to workers that are too close to the miner. Warnings are also provided to the operator of the machine. Provisions are made to immobilize the equipment until personnel were able to reach a safer position.
The magnetic fields used in the '471 patent system oscillate at low frequencies and can be effectively used to mark off silent zones. Although the maximum practical range of such low frequency magnetic fields may be less than 50 feet in most applications, which is more than is needed or desirable for most equipments. Typical haul trucks would probably be best served with a warning zone in the range of 20-30 feet and a danger zone in the range of 10-15 feet. In some applications, such as remotely controlled continuous miners, it is necessary for the operator to remain within a range of 10-25 feet much of the time in order to maintain good visual contact with the machine and the immediate surroundings. In underground mines, the low frequency magnetic fields pass through earth formations unimpeded so that a worker that is around a corner, not in line of sight, or otherwise obstructed, will still be visible to the marker field. These magnetic fields do not radiate from antennas but simply expand and contract around the element that produces them, and are well suited for marking boundaries between silent zones and warning zones.
There are situations where it is desirable for workers to be close to one part of the equipment but not close to another part of the equipment. An example is a truck that is backing up. A worker at the side of the truck may be at a very low risk or possibly no risk at all; yet, a worker behind the truck may be at a very high risk. Magnetic fields that extend far enough behind the truck to provide the needed protection, however, will generally also produce a larger than desired field to the sides of the truck. Therefore, there is a need to be able to shape a marker field to define areas where workers need to be positioned, and/or areas that present acceptable or lowered safety risks.